Based on quantum mechanical tunneling of electrons in field emission, a new model has been described to explain the saturation of emission current for carbon nanotube (CNT) emitters. The formulation reveals the effect of geometrical aspect ratio of CNT and the effect of high electric field for non-Fowler-Nordheim (F-N) behavior in field emission characteristics. It is found that for a CNT of aspect ratio similar to 10(3), the tunneling barrier converges to a constant square potential well of a small thickness, at applied electric field >= 3 x 10(6) V/m, resulting in field independent emission current. (C) 2010 Elsevier B.V. All rights reserved.